Sea levels could rise more than 3 feet by 2100, according to new study

Scientists say the consequences of sea-level rise could be 'horrible.'

In 2007, the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) predicted that global warming could cause sea levels to rise between 7 and 23 inches by the end of the century. A new study published this week says the actual levels of sea rise could be much higher — up to three feet higher.

The consequences of sea level rise at that proportion would be "horrible," the study's co-author told NBC News. Millions of people living in coastal areas would be displaced and mega-cities such as New York City and Tokyo would need to spend billions of dollars to improve their infrastructure to avoid flooding.

The study was published Jan. 6 in the journal Nature Climate Change. The paper claims that the IPCC report — which is in the process of being updated — did not fully anticipate the risk of melting Antarctic and Greenland glacial ice sheets and therefore underestimates the risk. The authors, J.L. Bamber and W.P. Aspinall of the University of Bristol in the U.K., pooled the findings of other researchers and came up with new estimates for sea level rise by 2100. Depending on the climate-change scenario, they predict that sea levels could rise between 11.4 and 33 inches (there was only a 5 percent chance of reaching this level, according to the experts' data). If rising temperatures also cause ocean waters to expand, the total sea rise could be more than three feet.

Bamber and Aspinall prepared this paper by polling 26 glaciologists around the world in 2010 and 2012. Bamber told NBC News that this technique, which has been used for earthquakes and volcanoes but not previously for climate change, is "a lot more than an opinion poll" since it rigorously analyzes ice sheet modeling and other information.

"This is the first study of its kind on ice sheet melting to use a formalized mathematical pooling of experts' opinions," Bamber said in a news release. "It demonstrates the value and potential of this approach for a wide range of similar problems in climate change research, where past data and current numerical modeling have significant limitations when it comes to forecasting future trends and patterns."

Bamber told NBC News that their study offers higher estimates than will be contained in the fifth IPCC assessment report, which is currently in the works. He is also serving as a reviewer of the IPCC report.

What the new paper does not answer is the reason why ice sheets have started melting faster than predicted, which could be because of climate change or a long-term trend. "Expert opinion is shown to be both uncertain and undecided," the authors write in the paper.

The study was funded by a European Union program called ice2sea, which has been set up to estimate the "future contribution of continental ice to sea-level rise."